It's actually the old jack, not the ensign. It would be flown at the bow (pointy end up front

) of the vessel, while in harbour. Even though that flag was technically reserved for ceremonial purposes right up until 1945, to be flown at Hitler's discretion, the proper flag to display for a boat built in 1941 and used by the Kriegsmarine between '42 and '45 would of course be the German War Ensign of the Third Reich. I guess someone figured they might possibly offend a few faint hearts with the truth, I dunno.
It may not be politically correct, but it's sure as hell historically correct. Keep it as such: